Low capacitance bidirectional transient voltage suppressor

ABSTRACT

A bidirectional transient voltage suppressor (TVS) circuit for data pins of electronic devices includes two sets of steering diodes and a diode triggered clamp device in some embodiment. In other embodiments, a bidirectional transient voltage suppressor (TVS) circuit for data pins of electronic devices includes two sets of steering diodes with a clamp device merged with a steering diode in each set. The TVS circuit is constructed to realize low capacitance at the protected nodes and improved clamping voltage for robust protection against surge evens. In some embodiments, the TVS circuit realizes low capacitance at the protected nodes by fully or almost completely depleting the P-N junction connected to the protected nodes in the operating voltage range. In this manner, the TVS circuit does not present undesirable parasitic capacitance to the data pins being protected, especially when the data pins are applied in high speed applications.

CROSS REFERENCE TO OTHER APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 16/447,704, entitled LOW CAPACITANCE BIDIRECTIONAL TRANSIENTVOLTAGE SUPPRESSOR, filed Jun. 20, 2019, which is a continuation of U.S.patent application Ser. No. 16/045,570, entitled LOW CAPACITANCEBIDIRECTIONAL TRANSIENT VOLTAGE SUPPRESSOR, filed Jul. 25, 2018, nowU.S. Pat. No. 10,373,947, issued Aug. 6, 2019, which is a continuationof U.S. patent application Ser. No. 15/605,662, entitled LOW CAPACITANCEBIDIRECTIONAL TRANSIENT VOLTAGE SUPPRESSOR, filed May 25, 2017, now U.S.Pat. No. 10,062,682, issued Aug. 28, 2018, which patents and patentapplications are incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

Voltages and current transients are major causes of integrated circuitfailure in electronic systems. Transients are generated from a varietyof sources both internal and external to the system. For instance,common sources of transients include normal switching operations ofpower supplies, AC line fluctuations, lightning surges, andelectrostatic discharge (ESD).

Transient voltage suppressors (TVS) are commonly employed for protectingintegrated circuits from damages due to the occurrences of transients orover-voltage conditions at the integrated circuit. Over-voltageprotection are important for consumer devices or the Internet of Thingsdevices as these electronic devices are exposed to frequent humanhandling and, as a result, may be susceptible to ESD or transientvoltage events that may damage the devices.

In particular, the power supply pins and the data pins of the electronicdevices both require protection from over-voltages conditions due to ESDevents or switching and lightning transient events. Typically, the powersupply pins need high surge protection but can tolerate protectiondevices with higher capacitance. Meanwhile, the data pins, which mayoperate at high data speed, requires protection devices that providesurge protection with low capacitance so as not to interfere with thedata speed of the protected data pins.

Existing TVS protection solution applied to input/output (I/O) terminalsin high speed applications exist both in vertical and lateral type ofsemiconductor circuit structures. In conventional verticalunidirectional structures, the I/O current during ESD flows from highside and low side steering diode vertically to ground. However, whenthese vertical structures are used for bidirectional TVS, the I/Ocurrent flows vertically and then laterally to ground via the second I/Oterminal. Due to the lateral current flow, the parasitic resistancebetween vertical and lateral current flow path increases which degradesthe clamping voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1 is a circuit diagram of a bidirectional TVS protection circuit insome embodiments of the present invention.

FIG. 2 is a circuit diagram of a bidirectional TVS protection circuit inembodiments of the present invention.

FIG. 3 is a circuit diagram of a bidirectional TVS protection circuit inembodiments of the present invention.

FIG. 4 illustrates the current-voltage characteristics of thebidirectional TVS circuit in embodiments of the present invention.

FIG. 5, which includes FIG. 5a , is a cross-sectional view of a part ofthe TVS circuit of FIG. 2 in embodiments of the present invention.

FIG. 6, which includes FIG. 6a , is a cross-sectional view of a part ofthe TVS circuit using a merged diode/clamp device structure inembodiments of the present invention.

FIG. 7, which includes FIG. 7a , is a cross-sectional view of a part ofthe TVS circuit using a merged diode/clamp device structure in alternateembodiments of the present invention.

FIG. 8, which includes FIG. 8a , is a cross-sectional view of a part ofthe TVS circuit using a merged diode/clamp device structure in alternateembodiments of the present invention.

FIG. 9, which includes FIG. 9a , is a cross-sectional view of a part ofthe TVS circuit using a merged diode/clamp device structure in alternateembodiments of the present invention.

FIG. 10, which includes FIG. 10a , is a cross-sectional view of a partof the TVS circuit using a merged diode/clamp device structure inalternate embodiments of the present invention.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; a composition of matter; a computerprogram product embodied on a computer readable storage medium; and/or aprocessor, such as a processor configured to execute instructions storedon and/or provided by a memory coupled to the processor. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention. Unless stated otherwise, a component such as aprocessor or a memory described as being configured to perform a taskmay be implemented as a general component that is temporarily configuredto perform the task at a given time or a specific component that ismanufactured to perform the task. As used herein, the term ‘processor’refers to one or more devices, circuits, and/or processing coresconfigured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

In embodiments of the present invention, a bidirectional transientvoltage suppressor (TVS) circuit includes two sets of steering diodesand a diode triggered clamp device. In other embodiments of the presentinvention, a bidirectional transient voltage suppressor (TVS) circuitincludes two sets of steering diodes with a clamp device merged with asteering diode in each set. The bidirectional TVS circuit of the presentinvention is constructed to realize low capacitance at the protectednodes during the blocking mode and realize improved clamping voltage forrobust protection against over-voltage transient events. Morespecifically, in some embodiments, the TVS circuit realizes lowcapacitance at the protected nodes by fully or almost completelydepleting the P-N junction connected to the protected nodes in theoperating voltage range of the protected nodes. In this manner, the TVScircuit does not present undesirable parasitic capacitance to data pinsbeing protected, especially when the data pins are used in high speedapplications.

In the present description, a transient voltage suppressor (TVS) circuitrefers to a protection circuit to protect a protected node fromover-voltage transient conditions, such as voltage surges or voltagespikes. The TVS circuit operates by shunting the excess current from theprotected node when a surge voltage exceeding the breakdown voltage ofthe TVS circuit is applied to the protected node. The TVS circuitincludes a clamp device for clamping the voltage at the protected nodeat a clamping voltage much lower than the voltage value of the voltagesurge while conducting the surge current. A TVS circuit can be either aunidirectional device or a bidirectional device. A unidirectional TVShas an asymmetrical current-voltage characteristic and is typically usedfor protecting circuit nodes whose signals are unidirectional—that is,the signals are always above or below a certain reference voltage, suchas ground. For example, a unidirectional TVS may be used to protect acircuit node whose normal signal is a positive voltage from 0V to 5V.

On the other hand, a bidirectional TVS has a symmetrical current-voltagecharacteristics and is typically used for protecting circuit nodes whosesignals are bidirectional or can have voltage levels both above andbelow the reference voltage, such as ground. For example, abidirectional TVS may be used to protect a circuit node whose normalsignal varies symmetrically above and below ground, such as from −12V to12V. In this case, the bidirectional TVS protects the circuit node froma surge voltage that goes below −12V or above 12V.

In operation, the TVS circuit is in a blocking mode and isnon-conductive except for possible leakage current when the voltage atthe protected node is below the breakdown voltage of the TVS circuit,sometimes referred to as a reverse standoff voltage. That is, when thevoltage at the protected node is within the normal voltage range for theprotected node, the TVS circuit is non-conductive and is in blockingmode. However, during the blocking mode, the TVS circuit presents acapacitance to the protected node. When the protected node is associatedwith a high speed data pin, the capacitance of the TVS circuit in theblocking mode or non-conductive mode should be low so as not to impedethe high speed operation of the data pin.

In some embodiments, the bidirectional TVS circuit of the presentinvention realizes a low capacitance value of less than 0.2 pf in theblocking mode. The low capacitance TVS circuit of the present inventioncan be advantageously applied to protect high-speed data pins orinput-output (I/O) terminals in high speed electronic applications, suchas data pins in USB3.1 data bus, HDMI-2.0 data bus, or V by One cables.

The bidirectional TVS circuit of the present invention realizes manyadvantages over conventional TVS circuits. First, the TVS circuit of thepresent invention is constructed to ensure that the current path of thesurge current flows in the lateral direction only through thesemiconductor device structure of the TVS circuit. The lateral currentflow improves the clamping voltage of the TVS circuit by reducing theresistance in the current path. For bi-directional TVS circuits, the TVScircuit of the present invention with lateral current flow realizesreduced resistance as compared to a vertical TVS structure or a TVScircuit with a vertical current flow. Second, the breakdown voltage ofthe TVS circuit can be tailored to a desired value by adjusting thejunction breakdown voltage of the clamp device, or adjusting thethreshold voltage of the MOSFET device in the clamp circuit, or by usinga dv/dt trigger. In some embodiments, the TVS circuit can optimize thecapacitance versus clamping voltage trade-off by adjusting the spacingbetween the anode and cathode regions of the clamp device. Third, insome embodiment, the reverse standoff voltage—or the holding voltage—ofthe TVS circuit can be adjusted by varying the emitter to baseresistance of bipolar transistors forming the clamp device.

FIG. 1 is a circuit diagram of a bidirectional TVS protection circuit inembodiments of the present invention. Referring to FIG. 1, a TVS circuit10 includes two sets of steering diodes coupled to provide surgeprotection for two input-output (I/O) terminals I/O1 and I/O2. Each setof steering diodes include a high-side steering diode and a low-sidesteering diode. More specifically, a high-side steering diode DH1 and alow-side steering diode DL1 are coupled to the I/O terminal I/O1 as theprotected node. Meanwhile, a high-side steering diode DH2 and a low-sidesteering diode DL2 are coupled to the I/O terminal I/O2 as the protectednode. The I/O terminal I/O1 is connected to the anode of the high-sidesteering diode DH1 and to the cathode of the low-side steering diodeDL1. Similarly, the I/O terminal I/O2 is connected to the anode of thehigh-side steering diode DH2 and to the cathode of the low-side steeringdiode DL2. The cathode terminals of diodes DH1 and DH2 are connected toa node N1. The anode terminals of diodes DL1 and DL2 are connected to anode N2.

The TVS circuit 10 also includes a clamp circuit 12 as the clamp device.The clamp circuit 12 includes a diode DC1 having a cathode connected tonode N1 and an anode connected to node N2 and a silicon controlledrectifier (SCR) DC2 having an anode connected to node N1 and a cathodeconnected to node N2. The clamp circuit 12 operates to clamp the voltageat nodes N1 and N2 when a zap voltage is applied to the I/O terminalswhile allowing the zap current to flow through the TVS circuit from oneI/O terminal to the other I/O terminal. In the bidirectional TVS circuit10, the nodes N1 and N2 are floating, that is, nodes N1 and N2 are notelectrically connected to or biased to any electrical potential.

In operation, when a positive zap is applied to I/O terminal I/O1 withrespect to I/O terminal I/O2, the current flows from terminal I/O1through diode DH1, SCR DC2, and then diode DL2 into terminal I/O2.Similarly, when a negative zap is applied to I/O terminal I/O1 withrespect to I/O terminal I/O2, which is equivalent to a positive zap onterminal I/O2 with respect to terminal I/O1, the current flows fromterminal I/O2 through diode DH2, SCR DC2, and then diode DL1 and intoterminal I/O1.

In other words, a positive zap voltage applied to either of the I/Oterminals will forward bias the high-side steering diode (DH1 or DH2) ofthe I/O terminal being zapped and when the zap voltage reaches orexceeds the breakdown voltage (BV) of the clamp circuit 12, the zapcurrent trigger the SCR DC2 and the SCR turns on to conduct current. Thezap current flows through the node N2 and forward bias the low-sidesteering diode (DL1 or DL2). The zap current then exits through theother I/O terminal. Thus, the node N1 will be more positively biasedrelative to node N2. A negative zap voltage applied to either of the I/Oterminals will result in the same current conduction operation as if apositive zap voltage is applied to the other I/O terminal.

FIG. 2 is a circuit diagram of a bidirectional TVS protection circuit inembodiments of the present invention. In particular, FIG. 2 illustratesthe construction of the clamp circuit in the TVS circuit of claim 1 insome embodiments. Referring to FIG. 2, the TVS circuit 20 includes twosets of steering diodes coupled to provide surge protection for twoinput-output (I/O) terminals I/O1 and I/O2, in the same manner as theTVS circuit 10 of FIG. 1. In TVS circuit 20, the clamp circuit 22 isformed by a diode-connected NMOS transistor M1 and a SCR formed by a PNPbipolar transistor Q1 and an NPN bipolar transistor Q2.

In operation, when a positive zap is applied to I/O terminal I/O1 withrespect to I/O terminal I/O2, the current flows from terminal I/O1through diode DH1, resistor R1, MOS transistor M1, and then diode DL2into terminal I/O2. As the zap voltage increases on terminal I/O1, thecurrent flowing in the aforementioned current path increases resultingin an increase in the voltage drop across resistor R1. When the voltagedrop across resistor R1 reaches about 0.7V which is enough to forwardbias the emitter-base junction of the PNP bipolar transistor Q1, thentransistor Q1 will enter into forward conduction and the current flowingthrough R2 will increase. When the voltage potential across resistor R2reaches 0.7V, then the emitter-base junction of the NPN bipolartransistor Q2 will be forward biased and at this point the SCR formed bytransistors Q1 and Q2 will be triggered and the SCR will conduct all thecurrent from node N1 to node N2 through the bipolar transistors Q1 andQ2.

Similarly, when a negative zap is applied to I/O terminal I/O1 withrespect to I/O terminal I/O2, which is equivalent to a positive zap onterminal I/O2 with respect to terminal I/O1, the current flows fromterminal I/O2 through diode DH2, into resistor R1, MOS transistor M1 andthen diode DL1 and into terminal I/O1. When the voltage potential acrossresistors R1 and R2 both reach 0.7V, as in the case of the positive zapdescribed above, then the SCR will turn on and conduct all current fromN1 to N2.

FIG. 3 is a circuit diagram of a bidirectional TVS protection circuit inembodiments of the present invention. In particular, the TVS circuit 30of FIG. 3 is constructed in the same manner as the TVS circuit 20 ofFIG. 2 except for the clamp circuit 32. Referring to FIG. 3, instead ofusing a diode-connected MOS transistor, the clamp circuit 32 in TVScircuit 30 uses an NMOS transistor M1 that is driven by a dv/dt triggercircuit. The dv/dt trigger circuit includes a capacitor C1 connected tonode N2 and a common node 14 and a resistor R3 connected between node N1and the common node 14. An inverter 12 inverts the state of the drivesignal coupled to the gate terminal of the NMOS transistor M1.

When the TVS circuit 30 is configured for bidirectional operation, theN1 and N2 nodes are floating. When either the I/O terminal I/O1 or I/O2is zapped, node N1 will get biased positive relative to node N2 and thedv/dt circuit will operate to trigger the clamp circuit 32 to clamp thevoltage across nodes N1 and N2. Alternately, the TVS circuit 30 can beconfigured for unidirectional operation. In unidirectional operation,the TVS circuit 30 may be applied to a system including an array of I/Oterminals and power supply pins—that is, the Vcc and ground pins. Inthat case, the node N1 will be coupled to the positive power supply Vccand the node N2 will be coupled to ground and the TVS circuit 30 willoperate to provide surge protection to the I/O terminals connectedthereto.

FIG. 4 illustrates the current-voltage characteristics of thebidirectional TVS circuit in embodiments of the present invention.Referring to FIG. 4, the TVS circuit of the present invention realizes asymmetrical current-voltage characteristics to provide bi-directionalprotection to a circuit node. The TVS circuit provides blocking when thevoltage level at the I/O terminal is below the operating voltageV_(RWM), for either positive or negative voltage polarity. The TVScircuit has very little leakage current in the blocking mode. When avoltage exceeding the breakdown voltage V_(BR) of the TVS circuit isapplied, the TVS circuit is triggered and the TVS circuit will snapback. The clamp circuit will clamp the voltage at the protected node(the I/O terminal) at the holding voltage (V_(Hold)). Zap current flowswith the voltage being held at the holding voltage until the surge eventis dissipated. As thus constructed, the TVS circuit of the presentinvention realizes bidirectional clamping with symmetricalcurrent-voltage characteristics for positive or negative zap voltages.

FIG. 5, which includes FIG. 5a , is a cross-sectional view of a part ofthe TVS circuit of FIG. 2 in embodiments of the present invention. Thecircuit diagram of the TVS circuit 20 of FIG. 2 is reproduced as FIG. 5ain FIG. 5. The cross-section of FIG. 5 illustrates circuit elements ofthe TVS circuit 20 including the high-side steering diode DH1, the clampdevice 22 and the low-side steering diode DL2. Referring to FIG. 5, theTVS circuit 20 is fabricated on an N+ substrate 50. In the presentembodiment, an N-type epitaxial layer 52 is formed on the N+ substrate50. In alternate embodiments, a P-type epitaxial layer can be formed onthe N+ substrate 50 instead. Then, N-type buried layer (NBL) 56 andP-type buried layer 54 are selectively formed on the N-type epitaxiallayer 52. In the present embodiment, the P-type buried layer 54 isformed as a Resurf P-type buried layer (R-PBL) having a lower dopinglevel than a conventional P-type buried layer. Then a second N-typeepitaxial layer 58 is formed on the N and P-type buried layers. Thesemiconductor structure for forming the TVS circuit is thus constructed.

In the present embodiment, trench isolation structures 60 are used todefine and isolate regions of the semiconductor structure for formingthe separate circuit elements. In the present embodiment, the trenchisolation structures 60 are formed as oxide lined trenches filled with apolysilicon layer and the trenches extend to the N+ substrate 50. Inother embodiments, the trench isolation structures 60 can be formed asoxide filled trenches. Furthermore, in some embodiments, when theepitaxial layer formed on the N+ substrate 50 is N-type, the trenchisolation structures can extend only up to the N-type epitaxial layer52. In alternate embodiments, when the epitaxial layer formed on the N+substrate 50 is a P-type epitaxial, the trench isolation structures willextend into the N+ substrate 50.

With the trench isolation structures 60 thus formed, regions in thesemiconductor structure for forming the high-side steering diode, theclamp device and the low-side steering diode are defined. In the presentembodiment, the high-side steering diode and the low-side steering areformed using the same diode device structure. In particular, thesteering diode is formed as a PN junction diode with the anode formed bya P-type region 64 and the cathode formed by the N-type epitaxial layer58. In the present embodiment, the P-type region 64 is formed using thedoping level typically used for a P-type compensation region and istherefore referred to as the PCOMP region 64. The doping level of thePCOMP region 64 is higher than the doping level of the N-type epitaxiallayer 58 but is lower than the heavily doped P+ region 62 used to makeohmic contact to the PCOMP region 64. In one example, the doping levelof the PCOMP region 64 is between from 1×10¹³ cm⁻³ to 1×10¹⁵ cm⁻³ and isa lower doping level in comparison to the doping level of the P-well 80which is about 1×10¹⁶ cm⁻³ to 5×10¹⁶ cm⁻³. A metal contact 87 is made inthe dielectric layer 86 to the P+ region 62 to form the anode terminal.Meanwhile, an N+ tungsten plug structure is used to make the ohmiccontact to the N-type epitaxial layer 58 as the cathode terminal. Inparticular, a shallow trench is formed in the N-epitaxial layer 58 and aheavily doped N+ region 68 is formed around the shallow trench. Thetrench is then filled with tungsten 66. A metal contact 89 is made tothe tungsten plug to form the cathode terminal.

For the high-side steering diode DH1, the anode terminal 87 is connectedto the I/O terminal I/O1 and the cathode terminal 90 is connected tonode N1. For the low-side steering diode DL2, the anode terminal 90 isconnected to node N2 and the cathode terminal 88 is connected to I/Oterminal I/O2. The high-side steering diode DH1 and the low-sidesteering diode DL2 are thus formed on the semiconductor structure.

The clamp device or clamp circuit of the TVS circuit 20 is formed asdiode-triggered SCR between the two steering diodes. In particular, theNMOS transistor M1 is formed by an N+ drain region 76 and an N+ sourceregion 78 formed in a P-well 80. A polysilicon gate 84 is formed abovethe channel region between the drain and source regions and is insulatedfrom the semiconductor layer (P-well 80 in N-Epitaxial 58) by a gateoxide layer. The gate 84 and the drain region 76 are electricallyshorted together so that the NMOS transistor M1 functions as adiode-connected MOS transistor.

The N+ drain region 76 is physically and electrically connected to anN-well region (NW) 72 formed adjacent to the N+ drain region. The N+drain region 76 is therefore electrically connected to the node N1through the N-well region 72 and a heavily doped N+ contact region 70.The N+ contact region 70 is connected to the cathode terminal 66 of thehigh-side steering diode DH1 by the metal contact 89. The N-well region72 provides a resistance between the drain region of transistor M1 andthe node N1, as depicted as the resistor R1 in the clamp circuit 20. Inone example, the doping level of the N-well region 72 is about 1×10¹⁶ tocm⁻³ to 5×10¹⁶ cm⁻³.

Meanwhile, the P-well 80 is electrically connected to the node N2through a heavily doped P+ contact region 82. A metal contact 90connects the P-well 80 to the node N2 and to the anode terminal (PCOMP)of the low-side steering diode DL2. The P-well 80 provides a resistancebetween the body region of transistor M1 and the node N2, as depicted asthe resistor R2 in the clamp circuit 20. In one example, the P-well 80has a doping level about 1×10¹⁶ cm⁻³ to 5×10¹⁶ cm⁻³. The N-wellresistance (R1) and the P-well resistance (R2) can be modified oradjusted by adjusting the doping levels or the length of the wellregions to adjust the resistance values.

The clamp circuit 20 further includes an integrated SCR device formed byheavily doped P+ region 74 and parasitic structures within NMOStransistor M1. In particular, a PNP bipolar transistor Q1 is formed byP+ region 74 as the emitter, N-well region 72 as the base, and P-well 80as the collector. An NPN bipolar transistor Q2 is formed by N-wellregion 72 as the collector, P-well 80 as the base and N+ region 78 asthe emitter. The base of the PNP transistor Q1 is the same as thecollector of the NPN transistor Q2 and the base of the NPN transistor Q2is the same as the collector of the PNP transistor Q1. As thusconfigured, a SCR is formed with the P+ region 74 as the anode and theN+ region 78 as the cathode and the P-well 80 as the control terminal ofthe SCR. The SCR conduction is triggered by the diode-connected NMOStransistor M1 which is connected across the base of PNP transistor Q1and the emitter of NPN transistor Q2. Once the SCR is triggered toconduct, the conduction is self-sustained without the biasing providedby the diode-connected NMOS transistor M1.

The TVS circuit as thus constructed realizes many advantages. First, thecurrent path of the surge current from one I/O terminal (e.g., I/O1) tothe other I/O terminal (e.g., I/O2) is primarily in the lateraldirection. The lateral current path for the surge current reducesundesirable parasitic resistance and improves the clamping voltagecharacteristics. Furthermore, the cathode of the steering diodes isformed using a trench N+ region which further improves clamping.

Second, the trigger voltage of the clamp circuit is determined by thethreshold voltage VT of the NMOS transistor M1. That is, when thegate-to-source voltage at the NMOS transistor M1 reaches the thresholdvoltage VT, the diode-connected NMOS M1 will conduct with current flowbetween the N+ drain 76 and N+ emitter/source 78. The current flow formsa base current flowing into PNP transistor Q1 which will trigger the SCRaction. Once the SCR action is triggered, the SCR will snap back andhold the voltage across nodes N1 and N2 at the holding voltage (FIG. 4)while continually conduct the surge current. Accordingly, the triggervoltage of the TVS circuit is determined by the threshold voltage of theNMOS transistor M1. The NMOS transistor M1 further provides dv/dtcontrol.

In one example, the threshold voltage of the NMOS transistor M1 is 3Vand the steering diode has a forward bias voltage drop of 0.7V. When apositive zap is applied to I/O terminal I/O1, the anode 64 (P+ 62/PCOMP64) is biased positive relative to node N1 and the high-side steeringdiode DH1 is forward biased and current flows from the diode to node N1.When the voltage at node N1 increases to a voltage of the forward biasvoltage drop (e.g. 0.7V) of the diode DH1 plus the threshold voltage VT(e.g. 3V) of the NMOS transistor M1, the NMOS transistor M1 is turned onand current will flow through the clamp device to the low-side steeringdiode DL2. The low-side steering diode is forward biased and currentflow from node N2 (anode) to the I/O terminal I/O2 (cathode) withanother forward bias voltage drop (0.7V) at the diode DL2. Thus, the TVScircuit 20 of FIG. 5 will be triggered to conduct current from terminalI/O1 to terminal I/O2 when a surge voltage applied to the I/O terminalexceeds the threshold voltage of the NMOS transistor plus two forwardbiased diode voltage drop, that is, V_(T)2FB. In the present example,the TVS circuit 20 will be triggered at a voltage of 3V+2*0.7V=4.4V.

Third, a salient characteristic of the steering diode structure usedherein is that the PN junction of the steering diode is completelydepleted at a bias voltage of 0V. Thus, the vertical parasiticcapacitance at the I/O terminal is substantially eliminated and the TVScircuit presents very low capacitance to the I/O terminal. Inparticular, the N-epitaxial layers 52, 58 and the R-PBL layer 54 arecompletely depleted to form a long vertical depletion region from theanode terminal 87 to the N+ substrate 50 of the semiconductor structure.In this manner, the parasitic capacitance as seen by the I/O terminal issubstantially reduced. The TVS circuit 20 thus provides surge protectionwith low capacitance and low leakage during blocking mode at theinput-output terminals.

In the present embodiment, the lightly doped PCOMP region 64 is used toenhance the depletion from the top surface of the semiconductorstructure at 0V bias voltage. As it is well understood, a depletionregion extends further in a lightly doped region than a heavily dopedregion. Thus, by using a more lightly doped P-type region 64 as theanode, it is possible to ensure that the entire vertical region of thePN junction is depleted to cut down on the parasitic capacitance. Inother embodiments, the PCOMP region 64 may be omitted and the PNjunction of N-Epitaxial layer 58 and the R-PBL layer 54 may still besufficiently depleted to cut down the parasitic capacitance.

In the semiconductor structured used to fabricate the TVS circuit ofFIG. 2, the steering diodes and the clamp device are formed separately,each in an active region defined by the trench isolation structures 60.In other embodiments, the TVS circuit can be reduced in size by mergingthe clamp device with one of the steering diode. FIG. 6, which includesFIG. 6a , is a cross-sectional view of a part of the TVS circuit using amerged diode/clamp device structure in embodiments of the presentinvention. FIG. 6a illustrates the circuit diagram of the TVS circuit100 using the merged diode/clamp device structure in some embodiments.Referring to FIG. 6a , the TVS circuit 100 includes two sets of steeringdiode and merged device coupled to provide surge protection for twoinput-output (I/O) terminals I/O1 and I/O2. More specifically, thehigh-side steering diode for I/O terminal I/O1 and the high-sidesteering diode for the I/O terminal I/O2 are each formed using a mergeddiode-clamp device, denoted as MDH1 and MDH2 respectively. A separateclamp device or clamp circuit is not used. Instead, the clamp device ismerged in with one of the steering diodes at each I/O terminal.

The cross-section of FIG. 6 illustrates circuit elements of the TVScircuit 100 including the merged diode-clamp device MDH1 and thelow-side steering diode DL2. Like elements in FIGS. 5 and 6 are givenlike reference numerals. Referring to FIG. 6, the low-side steeringdiode DL2 of FIG. 6 (and similarly the low-side steering diode DL1) isconstructed in a similar manner as the steering diode in the TVS circuit20 of FIG. 5. In the present embodiment, the diode structure forsteering diode DL2 is formed using an N+ region 68 only to make ohmiccontact with the cathode region (N-type epitaxial layer 58) to form thecathode terminal. In other embodiments, the N+ trench and tungsten plugstructure shown in FIG. 5 can be used instead of the N+ region 68.Furthermore, in the present embodiment, a P-type buried layer 53 isformed in additional to the Resurf P-type buried layer (R-PBL) 54. TheP-type buried layer 53 is more heavily doped than the R-PBL layer 54 andextends in the N-type epitaxial layer 52. The P-type buried layer 53 isused to cut down the parasitic NPN transistor gain between the N+emitter 68, the R-PBL base 54 and the N+ substrate-collector 50. TheP-type buried layer 53 increases the base doping of the parasitic NPNtransistor thus formed. In some examples, the P-type buried 53 may havea doping level of 1×10¹⁸ cm⁻³. The R-PBL layer 54 can have a dopinglevel 3 to 4 orders lower than the doping of the P-type buried layer 53.In operation, a vertical depletion region is formed from PCOMP region 64to the N-Epitaxial layer 52 at a bias voltage of 0V to reduce thevertical parasitic capacitance as seen by the I/O terminal I/O2.

In the merged diode/clamp device MDH1, the PN junction for the high-sidesteering diode is formed by the P-type region 64 (PCOMP) and the N-typeepitaxial layer 58. The P-type region 64 or PCOMP is a lightly dopedP-type region. The entire vertical junction area from PCOMP region 64 tothe N-Epitaxial layer 52 is depleted at 0V bias voltage to reduce thevertical parasitic capacitance as seen by the I/O terminal I/O1. Similarto the diode structure in FIG. 5, the PCOMP region 64 (as used in diodeDL2 or merged diode MDH1) is used to enhance the depletion from thesurface of the semiconductor structure at OV bias voltage. In otherembodiments, the PCOMP region 64 can be omitted.

Meanwhile, the clamp device, formed as a SCR, is integrated with thehigh-side steering diode. In particular, the anode of the SCR is formedby the P+ region 62 with PCOMP region 64 and is detached or separatedfrom the N-well region 72. That is, the anode of the SCR (P+ region62/PCOMP region 64) is not electrically or physically shorted to theN-well region 72. At a bias voltage of 0V, the PCOMP region 64 willoperate to completely deplete the N-well region 72 as well to reduce theparasitic capacitance.

The PNP bipolar transistor Q1 of the SCR is formed by the P+ region 62(with or without PCOMP region 64) as the emitter, N-well region 72 asthe base, and P-well 80 as the collector emitter. The NPN bipolartransistor Q2 of the SCR is formed by N-well region 72 as the collector,P-well 80 as the base and N+ region 78 as the emitter. The base of thePNP transistor Q1 is the same as the collector of the NPN transistor Q2and the base of the NPN transistor Q2 is the same as the collector ofthe PNP transistor Q1. As thus configured, a SCR is formed with the P+region 62 as the anode and the N+ region 78 as the cathode and theP-well 80 as the control terminal of the SCR. The anode of the SCR iselectrically connected to the I/O terminal I/O1 through the metalcontact 87 and the cathode of the SCR is electrically connected to thenode N1 through the metal contact 89. The P-well 80 is electricallyconnected to the node N1 through the heavily doped P+ contact region 82.

The trigger voltage of the TVS circuit 100 is determined by thebreakdown voltage of the N+ region 76 and the P-well 80. When a surgevoltage is applied to the I/O terminal I/O1, the high-side steeringdiode DH1 is forward biased and current flows to the N-well region 72and to N+ region 76. When sufficient current through N-well region 72and N+ region 76 to the P-well 80, the PNP transistor Q1 of the SCR istriggered and starts to conduct. Continued current flow will furthertrigger the NPN transistor Q2 of the SCR to turn on the SCR action toconduct the surge current. The current flow through the mergeddiode/clamp device MDH1 and the low-side steering diode DL2 is primarilyin the lateral direction to ensure low resistance and improved clamping.

In the above-described embodiments, the TVS circuit is fabricated in asemiconductor structure using N+ substrate. In other embodiments, a P+substrate may be used with corresponding changing to the polarities ofother layers. FIG. 7, which includes FIG. 7a , is a cross-sectional viewof a part of the TVS circuit using a merged diode/clamp device structurein alternate embodiments of the present invention. The TVS circuit 110of FIG. 7a is the same as the TVS circuit 100 of FIG. 6a . However, theTVS circuit 110 of FIG. 7 is fabricated on a P+ substrate 51 instead ofan N+ substrate used in FIG. 6.

Referring to FIG. 7, the TVS circuit 110 is fabricated on the P+substrate 51. A P-type epitaxial layer 55 is formed on the P+ substrate51. Then, N-type buried layer (NBL) 56 and Resurf N-type buried layer(R-NBL) 57 are selectively formed on the P-type epitaxial layer 55.Then, a second P-type epitaxial layer 59 is formed on the N and P-typeburied layers. The semiconductor structure for forming the TVS circuitis thus constructed.

In the TVS circuit 110 of FIG. 7, the low-side steering diode DL2 (andsimilarly for the low-side steering diode DL1) is formed in an activeregion isolated by trench isolation structures 60. The steering diode isformed as a PN junction diode with the anode formed by the P+ region 74in the P-Epitaxial layer 59 and the cathode formed by an N-type region65. A metal contact 89 is made through the dielectric layer 86 to the P+region 74 to form the anode terminal. In the present embodiment, theN-type region 65 is formed using the doping level typically used for anN-type compensation region and is therefore referred to as the NCOMPregion 65. The doping level of the NCOMP region 65 is higher than thedoping level of the P-type epitaxial layer 59 but is lower than theheavily doped N+ region 68 used to make ohmic contact to the NCOMPregion 65. In one example, the NCOMP region 65 has a doping level of1×10¹³ cm⁻³ to 1×10¹⁵ cm⁻³ and the N-well region 72 has a doping levelabout 1×10¹⁶ cm⁻³ to 5×10¹⁶ cm⁻³. A metal contact 88 is made in thedielectric layer 86 to the N+ region 68 to form the cathode terminal. Inoperation, a vertical depletion region is formed from NCOMP region 65 tothe P-Epitaxial layer 55 at a bias voltage of 0V to reduce the verticalparasitic capacitance as seen by the I/O terminal I/O2. The R-NBL layer57 is completely depleted or partially depleted at the bias voltage of0V by the NCOMP region 65 due to the higher doping level of the R-NBLlayer. Even if the R-NBL layer 57 is only partially depleted, theterminal I/O2 sees two capacitors in series which presents a smallcapacitance.

In the merged diode/clamp device MDH1, the PN junction for the high-sidesteering diode is formed by the N-well region 72 and a P-well region 81.The entire vertical junction area from N-well region 72 to theP-Epitaxial layer 55 is depleted at 0V bias voltage to reduce thevertical parasitic capacitance as seen by the I/O terminal I/O1.

Meanwhile, the clamp device, formed as a SCR, is integrated with thehigh-side steering diode. More specifically, the PNP bipolar transistorQ1 of the SCR is formed by the P+ region 74 as the emitter, N-wellregion 72 as the base, and P-well region 81 as the collector. The P-wellregion 81 is more heavily doped than the P-Epitaxial layer 59 but morelightly doped than the P+ region 74. In one example, the P-well region81 has a doping level about 1×10¹⁶ cm⁻³ to 5×10¹⁶ cm⁻³. The NPN bipolartransistor Q2 of the SCR is formed by N-well region 72 as the collector,P-well region 81 as the base and N+ region 78 as the emitter. The baseof the PNP transistor Q1 is the same as the collector of the NPNtransistor Q2 and the base of the NPN transistor Q2 is the same as thecollector of the PNP transistor Q1. As thus configured, a SCR is formedwith the P+ region 74 as the anode and the N+ region 78 as the cathodeand the P-well region 81 as the control terminal of the SCR. The anodeof the SCR is electrically connected to the I/O terminal I/O1 throughthe metal contact 87 and the cathode of the SCR is electricallyconnected to the node N1 through the metal contact 89. The P-well region81 is electrically connected to the node N1 through the heavily doped P+contact region 82.

In other embodiments, the TVS circuit of FIG. 7 can be made without theN-type buried layer 56. FIG. 8, which includes FIG. 8a , is across-sectional view of a part of the TVS circuit using a mergeddiode/clamp device structure in alternate embodiments of the presentinvention. The TVS circuit 120 of FIG. 8a is the same as the TVScircuits 100 and 110 of FIGS. 6a and 7a . Referring to FIG. 8, the TVScircuit 120 of FIG. 8 is fabricated in the same manner as the TVScircuit 110 of FIG. 7 except that the N-type buried layer 56 is omitted.In this case, the N-well will completely deplete the vertical region ofthe P-type epitaxial layers 55 and 59 to reduce the vertical parasiticcapacitance.

FIG. 9, which includes FIG. 9a , is a cross-sectional view of a part ofthe TVS circuit using a merged diode/clamp device structure in alternateembodiments of the present invention. FIG. 9a illustrates the circuitdiagram of the TVS circuit 130 using the merged diode/clamp devicestructure in some embodiments. Referring to FIG. 9a , the TVS circuit130 includes two sets of steering diode and merged device coupled toprovide surge protection for two input-output (I/O) terminals I/O1 andI/O2. More specifically, the low-side steering diode for I/O terminalI/O1 and the low-side steering diode for the I/O terminal I/O2 are eachformed using a merged diode-clamp device, denoted as MDL1 and MDL2respectively. A separate clamp device or clamp circuit is not used.Instead, the clamp device is merged in with one of the steering diodesat each I/O terminal.

The cross-section of FIG. 9 illustrates circuit elements of the TVScircuit 130 including the high-side steering diode DH1 and the mergeddiode-clamp device MDL2. Referring to FIG. 9, the TVS circuit 130 isfabricated on an N+ substrate 50. In the present embodiment, an N-typeepitaxial layer 52 is formed on the N+ substrate 50. In alternateembodiments, a P-type epitaxial layer can be formed on the N+ substrate50 instead. Then, N-type buried layer (NBL) 56 and a Resurf P-typeburied layer (R-PBL) 54 are selectively formed on the N-type epitaxiallayer 52. Then a second N-type epitaxial layer 58 is formed on the N andP-type buried layers. The semiconductor structure for forming the TVScircuit is thus constructed. The semiconductor structure used in FIG. 9is similar to the one used in FIGS. 5 and 6.

In TVS circuit 130, the high-side steering diode DH1 (and similarly thehigh-side steering diode DH2) is constructed in a similar manner as thesteering diode in the TVS circuit 100 of FIG. 6, but without the P-typeburied layer 53 used in TVS circuit 100 of FIG. 6. In operation, avertical depletion region is formed from PCOMP region 64 to theN-Epitaxial layer 52 at a bias voltage of 0V to reduce the verticalparasitic capacitance as seen by the I/O terminal I/O1.

In the merged diode/clamp device MDL2, the PN junction for the low-sidesteering diode is formed by the P+ region 74 to the N-well region 72.The entire vertical junction area from N-well region 72 to theN-Epitaxial layer 52 is depleted at 0V bias voltage to reduce thevertical parasitic capacitance as seen by the I/O terminal I/O1.

Meanwhile, the clamp device, formed as a SCR, is integrated with thelow-side steering diode. More specifically, the PNP bipolar transistorQ1 of the SCR is formed by the P+ region 74 as the emitter, the N-wellregion 72 as the base, and the P-well region 80 as the collector. TheNPN bipolar transistor Q2 of the SCR is formed by N-well region 72 asthe collector, P-well region 80 as the base and N+ region 78 as theemitter. The base of the PNP transistor Q1 is the same as the collectorof the NPN transistor Q2 and the base of the NPN transistor Q2 is thesame as the collector of the PNP transistor Q1. As thus configured, aSCR is formed with the P+ region 74 as the anode and the N+ region 78 asthe cathode and the P-well region 80 as the control terminal of the SCR.The anode of the SCR is electrically connected to node N1 through themetal contact 89 and the cathode of the SCR is electrically connected tothe I/O terminal I/O2 through the metal contact 88. The P-well region 80is electrically connected to the I/O terminal I/O2 through the heavilydoped P+ contact region 82.

FIG. 10, which includes FIG. 10a , is a cross-sectional view of a partof the TVS circuit using a merged diode/clamp device structure inalternate embodiments of the present invention. The TVS circuit 140 ofFIG. 10a is the same as the TVS circuit 130 of FIG. 9a . Referring toFIG. 10, the TVS circuit 140 of FIG. 10 is fabricated in a similarmanner as the TVS circuit 130 of FIG. 9 except for the use of P-typeepitaxial layers 55 and 59 instead of the N-type epitaxial layers 52 and58 in FIG. 9. In particular, the TVS circuit 110 is fabricated on the N+substrate 50. The P-type epitaxial layer 55 is formed on the N+substrate 50. Then, N-type buried layer (NBL) 56 and Resurf N-typeburied layer (R-NBL) 57 are selectively formed on the P-type epitaxiallayer 55. Then, a second P-type epitaxial layer 59 is formed on the Nand P-type buried layers. The semiconductor structure for forming theTVS circuit is thus constructed.

In TVS circuit 140, the high-side steering diode DH1 (and similarly forthe low-side steering diode DL2) is formed in an active region isolatedby trench isolation structures 60. The steering diode is formed as a PNjunction diode with the anode formed by the P+ region 62 in theP-Epitaxial layer 59 and the cathode formed by an N-type region 65. Ametal contact 87 is made through the dielectric layer 86 to the P+region 74 to form the anode terminal. In the present embodiment, theN-type region 65 is formed using the doping level typically used for anN-type compensation region and is therefore referred to as the NCOMPregion 65. The doping level of the NCOMP region 65 is higher than thedoping level of the P-type epitaxial layer 59 but is lower than theheavily doped N+ region 68 used to make ohmic contact to the NCOMPregion 65. In one example, the NCOMP region 65 has a doping level of1×10¹³ cm⁻³ to 1×10¹⁵ cm⁻³ and the N-well region 72 has a doping levelabout 1×10¹⁶ cm⁻³ to 5×10¹⁶ cm⁻³. A metal contact 89 is made in thedielectric layer 86 to the N+ region 68 to form the cathode terminal. Inoperation, a vertical depletion region is formed from NCOMP region 65 tothe P-Epitaxial layer 55 at a bias voltage of 0V to reduce the verticalparasitic capacitance as seen by the I/O terminal I/O2. The R-NBL layer57 is completely depleted at the bias voltage of 0V by the NCOMP region65.

In the merged diode/clamp device MDL2, the PN junction for the low-sidesteering diode is formed by the N-well region 72 to the P-well region81. The entire vertical junction area from N-well region 72 to theP-Epitaxial layer 55 is depleted at 0V bias voltage to reduce thevertical parasitic capacitance as seen by the I/O terminal I/O1.

Meanwhile, the clamp device, formed as a SCR, is integrated with thelow-side steering diode. More specifically, the PNP bipolar transistorQ1 of the SCR is formed by the P+ region 74 as the emitter, N-wellregion 72 as the base, and a P-well region 81 as the collector. TheP-well region 81 is more heavily doped than the P-Epitaxial layer 59 butmore lightly doped than the P+ region 74. In one example, the P-wellregion 81 has a doping level about 1×10¹⁶ cm⁻³ to 5×10¹⁶ cm⁻³. The NPNbipolar transistor Q2 of the SCR is formed by N-well region 72 as thecollector, the P-well region 81 as the base and N+ region 78 as theemitter. The base of the PNP transistor Q1 is the same as the collectorof the NPN transistor Q2 and the base of the NPN transistor Q2 is thesame as the collector of the PNP transistor Q1. As thus configured, aSCR is formed with the P+ region 74 as the anode and the N+ region 78 asthe cathode and the P-well region 81 as the control terminal of the SCR.The anode of the SCR is electrically connected to the node N1 throughthe metal contact 89 and the cathode of the SCR is electricallyconnected to the I/O terminal I/O2 through the metal contact 88. TheP-well region 81 is electrically connected to the I/O terminal I/O2through the heavily doped P+ contact region 82.

In the embodiment shown in FIG. 10, an N+ substrate is used. Inalternate embodiments, a P+ substrate can be used, as shown by theembodiment in FIG. 7.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A bidirectional transient voltage suppressing(TVS) device comprising: a semiconductor layer comprising a firstepitaxial layer, one or more buried layers formed on the first epitaxiallayer, and a second epitaxial layer formed on the one or more buriedlayers; a plurality of active regions formed in the semiconductor layer,the active regions being isolated from each other by isolationstructures; a first diode formed in a first active region of theplurality of active regions, the first active region comprising a firstburied layer formed on the first epitaxial layer, the first diodecomprising a first region formed in the second epitaxial layer, whereinthe first region, the first buried layer and the first and secondepitaxial layers in the first active region are depleted at a biasvoltage of zero volt, reducing a vertical parasitic capacitance at theanode of the first diode; and a merged diode/clamp device formed in asecond active region of the plurality of active regions, the secondactive region comprising a second buried layer formed on the firstepitaxial is layer, the merged diode/clamp device comprising a seconddiode integrated with a silicon controlled rectifier (SCR), the seconddiode having a cathode being formed in a second region of a firstconductivity type formed in the second epitaxial layer and having ananode being formed in a third region of a second conductivity type,opposite to the first conductivity type, formed in the second region,and the SCR having an anode formed in the third region and a cathodeformed in a fourth region of the first conductivity type formed in afifth region of the second conductivity type formed in the secondepitaxial layer, wherein the second region, the second buried layer andthe first and second epitaxial layers in the second active region aredepleted at a bias voltage of zero volt, reducing a vertical parasiticcapacitance at the anode of the second diode, wherein the anode of thefirst diode is coupled to a first protected node and the cathode of thesecond diode is coupled to a second protected node, the cathode of thefirst diode is coupled to the anode of the SCR, and wherein in responseto a voltage applied to one of the protected nodes exceeding a firstvoltage level, the SCR conducts and clamps the voltage at the respectiveprotected node at a clamping voltage.
 2. The bidirectional TVS device ofclaim 1, wherein: the semiconductor layer comprising the first epitaxiallayer of the first conductivity type, the first buried layer of thesecond conductivity type formed on the first epitaxial layer in thefirst active region, the second buried layer of the first conductivitytype formed on the first epitaxial layer in the second active region,and the second epitaxial layer of the first conductivity type formed onthe first and second buried layers; and the first region comprises adoped region of the second conductivity type formed in the secondepitaxial layer, the second epitaxial layer being a cathode and thefirst region being an anode of the first diode.
 3. The bidirectional TVSdevice of claim 2, wherein the second region of the merged diode/clampdevice comprises a first well of the first conductivity type formed inthe second epitaxial layer; the third region of the merged diode/clampdevice comprises a heavily doped region of the second conductivity typeformed in the first well; the fifth region of the merged diode/clampdevice comprise a second well of the second conductivity type formed inthe second epitaxial layer and adjacent the first well; and the fourthregion of the merged diode/clamp device comprises a heavily doped regionof the first conductivity type formed in the second well.
 4. Thebidirectional TVS device of claim 3, further comprising: a sixth regionof the first conductivity type being heavily doped and formed in contactwith the first well; and a seventh region of the second conductivitytype being heavily doped and formed in the second well.
 5. Thebidirectional TVS device of claim 3, further comprising: an eighthregion of the first conductivity type being heavily doped and formed inand in contact with the first well and the second well, a triggervoltage of the TVS device being indicative of a breakdown voltagebetween the eighth region and the second well.
 6. The bidirectional TVSdevice of claim 2, wherein the first region comprises a heavily dopedregion of the second conductivity type formed in a lightly doped regionof the second conductivity type, the lightly doped region and the firstburied layer being provided to deplete the first and second epitaxiallayers at the bias voltage of zero volt.
 7. The bidirectional TVS deviceof claim 2, wherein the first diode further comprises a nineth region ofthe first conductivity type and heavily doped, the nineth region beingformed in the second epitaxial layer and spaced apart from the firstregion.
 8. The bidirectional TVS device of claim 1, wherein: thesemiconductor layer comprising the first and second buried layers of afirst conductivity type formed on the first epitaxial layer of thesecond conductivity type, and the second epitaxial layer of the secondconductivity type formed on the first and second buried is layers; andthe first region of the first conductivity type formed in the secondepitaxial layer, the second epitaxial layer being an anode and the firstregion being a cathode of the first diode.
 9. The bidirectional TVSdevice of claim 8, wherein the second region of the merged diode/clampdevice comprises a first well of the first conductivity type formed inthe second epitaxial layer; the third region of the merged diode/clampdevice comprises a heavily doped region of the second conductivity typeformed in the first well; the fifth region of the merged diode/clampdevice comprise a second well of the second conductivity type formed inthe second epitaxial layer and adjacent the first well; and the fourthregion of the merged diode/clamp device comprises a heavily doped regionof the first conductivity type formed in the second well.
 10. Thebidirectional TVS device of claim 9, further comprising: a tenth regionof the first conductivity type being heavily doped and formed in contactwith the first well; and an eleventh region of the second conductivitytype being heavily doped and formed in the second well.
 11. Thebidirectional TVS device of claim 9, further comprising: a twelfthregion of the first conductivity type being heavily doped and formed inand in contact with the first well and the second well, a triggervoltage of the TVS device being indicative of a breakdown voltagebetween the eighth region and the second well.
 12. The bidirectional TVSdevice of claim 8, wherein the first region comprises a heavily dopedregion of the first conductivity type formed in a lightly doped regionof the first conductivity type, the lightly doped region and the firstburied layer being provided to deplete the first and second epitaxiallayers at the bias voltage of zero volt.
 13. The bidirectional TVSdevice of claim 8, wherein the first diode further comprises athirteenth region of the second conductivity type and heavily doped, thethirteenth region being formed in the second epitaxial layer and spacedapart from the first region.
 14. The bidirectional TVS device of claim1, further comprising a semiconductor substrate of the firstconductivity type and being heavily doped, the semiconductor layer beingformed on the semiconductor substrate.
 15. The bidirectional TVS deviceof claim 1, wherein each of the isolation structures comprises an oxidefilled trench isolation structure.
 16. The bidirectional TVS device ofclaim 1, wherein each of the isolation structures comprises a trenchisolation structure, each trench being lined with an oxide layer andfilled with a polysilicon layer.
 17. The bidirectional TVS device ofclaim 1, wherein the first conductivity type comprises N-typeconductivity and the second conductivity type comprises P-typeconductivity.